In its gleefully befogged first hour, " Pineapple Express" seems to be onto something new: It's a marijuana comedy that keeps shuffling genres, like a stoned blackjack dealer. James Franco is blissfully funny as Saul, the supplier who finds himself running for his barely cognizant life with steady customer Dale, played by Seth Rogen. Dale's a 25-year-old dating a high school senior. (He proudly sports a high school girl's wristwatch.) They're running because Dale witnessed a drug-related murder and then dropped a precious joint at the scene of the crime. Unsure whether they're in a comedy or a drama, Gary Cole and Rosie Perez play the ruthless criminals who want the witness dead. The film's title refers to a particularly rare and exquisite brand of weed.

At its sharpest, the script by Rogen and Evan Goldberg, who co-wrote "Superbad," recalls what made "Superbad" worth seeing: the sidewinding conversational riffs, the why-am-I-laughing? wordplay. When Dale explains to his dealer that he's a process server, he replies, "You're a servant? Like, a butler?" As written, that line could go either way, but Franco—fully invested in this doper's doper—makes it sing. It's tempting to say Franco knocks such stupid retorts out of the park, but that would imply a certain degree of focus and drive utterly lacking in Saul. Not since the cinematic cannabis heyday of Jeff Spicoli ("Fast Times at Ridgemont High," 1982) and "Withnail & I" (1987) has a fully baked stoner come to such worthy comic life on-screen.

Then, around the midpoint, "Pineapple Express" falls apart and keeps falling, and the comedy, spiced with considerable, unevenly effective violence in that first hour, goes out the window, and in comes all the gore and the bone-crunching. The director is David Gordon Green, who made one of my favorite films this year, "Snow Angels." His work (which includes the poetic "George Washington") has never been easily confined to one category or mood. But there's probably not a director alive who could make sense of this script's queasy blend of jokes and slaughter. Green shoots the fight sequences with rough edges and hand-held realism intact, and the realism is ... well, real. An early smackdown between Rogen and Danny McBride's belligerent idiot Red ends with the trashing of a perfectly good apartment; the scene grinds on well past its usefulness, and the injuries grow more wince-inducing, and before long you're thinking back on "Freebie and the Bean," another comedy that kept morphing into an accidental and deliberately acrid action picture.

For all that, Franco's on-screen rapport with Rogen is a fine thing. Certain lines keep coming back to me, lines that could've come only from truly talented writers. "Hey! I can see through my leg hole!" is one; Franco says it after he kicks through his own windshield during a vehicular chase sequence. Also, there's a throwaway bit with Franco attempting to buzz Rogen into his apartment that approaches perfection—the quintessence of pot humor, honoring the tradition of the "Dave's not here" routine (Cheech & Chong, for the uninitiated). "Pineapple Express" could care less about perfection; its mood swings and genre change-ups are deliberately messy. Few comedies recently have started so well and ended so poorly. Whether that first half is enough is entirely up to your receptivity to another pair of jolly Judd Apatow-sanctioned boy-men,taking time to smell the roses even as people are trying to kill them.

See the trailer and find local showtimes for "Pineapple Express."